1. Field of the Invention
The present invention relates to a method for manufacturing an electrode for a secondary battery such as a lithium secondary battery, and more specifically, to a method for manufacturing an electrode for a secondary battery obtained by depositing a thin film composed of active material on the current collector.
2. Related Art
In lithium secondary batteries which have been studied and developed enthusiastically in recent years, an electrode used therein has great influence on their battery properties such as charge/discharge voltage, charge/discharge cycle life characteristics, and storage characteristics. Therefore, it has been attempted to enhance the battery properties by improving active materials.
Using lithium metal as negative active material can realize a battery with high energy density both per weight and per volume; however, it causes the problem that lithium is deposited in the form of dendrite during a charge, which leads to an internal short-circuiting.
On the other hand, it is reported that another type of lithium secondary battery using, as an electrode, aluminum, silicon, tin, or the like which is electrochemically alloyed with lithium during a charge. Among these materials, silicon with a high theoretical capacity is promising as the negative electrode for a battery with a high capacity, and various types of secondary batteries with this negative electrode have been suggested (Japanese Patent Laid Open No. 10-255768). However, in these types of alloy electrodes, the alloy which is active material is pulverized during a charge/discharge and deteriorates current collecting properties, so sufficient cycle characteristics are not obtained.
We have already found out that a lithium secondary battery with a capacity as high as around 4000 mAh/g and with cycle characteristics good enough for practical use can be obtained by using, as an electrode, a microcrystalline silicon thin film or an amorphous silicon thin film formed on a copper foil by CVD or sputtering method (International Publication WO01/31720A1 and others)
We have also found out that in this electrode the silicon thin film must have excellent adhesion to the copper foil, which is the current collector, and that this adhesion is obtained by the copper element from the copper foil being diffused in the silicon thin film to form a solid solution. We have further found out that an intermetallic compound between silicon and copper formed from too much diffusion of the copper element into the silicon thin film causes a decrease in adhesion and capacity. Hence, in manufacturing such an electrode, it is important to control the diffusion of the current collector material into the thin film of active material.
The object of the present invention is to provide a method for manufacturing an electrode for a secondary battery with excellent cycle characteristics, in which the adhesion between a current collector and a thin film can be improved by controlling the diffusion of the current collector material into the thin film.
The present invention is a method for manufacturing an electrode for a secondary battery by depositing a thin film composed of active material on a current collector in which a surface-treated layer is formed, comprising the steps of: removing at least part of the surface-treated layer by etching the surface of the current collector with an ion beam or plasma in order to improve the diffusion of the current collector material into the thin film; and depositing the thin film on the surface of the current collector subjected to the etching step.
When the presence of a surface-treated layer suppresses the diffusion of the current collector material into the active material thin film, according to the present invention, it is possible to promote the diffusion of the current collector material into the thin film, thereby obtaining excellent adhesion.
As the current collector for a secondary battery such as a lithium secondary battery, a metallic foil such as a copper foil is generally used. The surface of the metallic foil is generally subjected to an antirust treatment in order to prevent corrosion. As an antirust treatment, chromate treatment, silane coupling treatment, and benzotriazol treatment are known. Even when such an antirust treatment is not applied, an oxide film may be formed in the surface of the metallic foil.
We have found that when an electrode for a secondary battery is manufactured by depositing a thin film composed of active material on a metallic foil by CVD or sputtering method, the metallic atoms or molecules with high energy collide with the surface of the current collector and the temperature of the current collector increases, making a component of the current collector diffuse into the thin film. In the area where the current collector material is diffused, a solid solution of the current collector material and the active material is formed, and the presence of the area with the solid solution improves the adhesion between the current collector and the thin film, thereby increasing the charge/discharge cycle characteristics.
The above-mentioned antirust-treated layer or oxide layer in the surface of the current collector suppresses the diffusion of the current collector material into the thin film so as to decrease the adhesion between the current collector and the thin film, making it easier for the thin film to delaminate from the current collector due to the stress resulting from expansion and shrinkage of the active material during a charge/discharge. According to the present invention, the antirust-treated layer or oxide film in the surface of the current collector can be removed as described above, and therefore it becomes possible to promote the diffusion of the current collector material into the thin film to obtain excellent adhesion.
On the other hand, there are cases that excessive diffusion of the current collector material into the thin film decreases the adhesion between the current collector and the thin film. For example, when a copper foil is used as the current collector, and a microcrystalline silicon thin film or an amorphous silicon thin film is provided thereon as a thin film of active material, if copper component is excessively diffused in the silicon thin film, an intermetallic compound is formed which may decrease the adhesion of the silicon thin film to the copper foil. Therefore, in such cases, it is preferable to control the etching degree of the surface-treated layer so as to improve the diffusion of the current collector material into the thin film within the range that the intermetallic compound of the active material and the current collector material is not formed.
In the etching step and the depositing step, it is preferable to control the temperature of the current collector in order to prevent the temperature from increasing to cause the current collector material to be diffused excessively. For example, in the etching step and the depositing step, it is preferable that the temperature of the current collector is controlled to be in the range of 20 to 250xc2x0 C.
The etching step and the depositing step of the present invention can be performed in the same reaction chamber because they can be performed in a low pressure. Therefore, in the present invention, the etching step and the depositing step are preferably performed in succession in the same reaction chamber. Thus, an electrode for a secondary battery can be manufactured effectively.
In the present invention, the method of depositing a thin film on the current collector is not restricted particularly; there are other methods of forming a thin film from a vapor phase, such as CVD, sputtering, vacuum evaporation, or thermal spraying method. It is also possible to use a method of forming a thin film from a liquid phase, such as plating method.
When the current collector material is not sufficiently diffused into the thin film after the formation of the thin film, a heat treatment may be applied.
When a silicon thin film is used as the thin film, copper element (Cu) can be used as element easily diffused in the silicon thin film. Therefore, when a silicon thin film is formed as the thin film, a copper foil can be preferably used as the current collector. Such a copper foil includes a rolled copper foil and an electrolytic copper foil. We have found that an electrolytic copper foil is high in surface activity, so that copper is easily diffused into the silicon thin film. In addition, a high surface roughness Ra makes larger the interface area between the foil and the silicon thin film, which contributes to an increase in adhesion of the silicon thin film. Thus, it is preferable to use an electrolytic copper foil, when a copper foil is used as the current collector.
It is also possible to use a metallic foil whose surface portion is made of copper or its alloy. For example, it may be used, as the current collector, a metallic foil such as a nickel foil on the surface of which a copper layer is provided.